In the name of Allah, Most Compassionate, Most Merciful,
Firstly, one of the conditions for a valid sale is that the object of sale must be in the ownership of the seller. As such, a person can not sell an item which has not lawfully come into his ownership. (Sahih al-Bukhari & major fiqh references).
Secondly, an item that was lost and found must be first advertised thoroughly until the owner is found. However, if after extensive search, the owner could not be found, then the item should be given in charity. If the person is poor himself, he can keep it for personal use. (Radd al-Muhtar, 6/433).
Combining the above two rulings, it becomes clear that it is (normally) impermissible to sell an item that was found, for the seller is not considered a rightful owner of it. Similarly, to purchase such items would not be permissible, and it would be necessary to return it to their rightful owners or give it in charity on their behalf.
Therefore, if one is fully aware that a particular item was lost and found, and it is being sold, then one should avoid purchasing the item.
And Allah knows best
